A conventional light-emitting device includes a substrate, two electrodes (an anode and a cathode), an emissive layer (EML) containing a material that emits light upon electron and hole recombination, one or more layers between the anode and the EML, and one or more layers between the cathode and the EML. The one or more layers between the anode and the EML may be hole transporting layers (HTLs), hole injection layers (HILs), or electron blocking layers (EBLs). The one or more layers between the cathode and the EML may be electron transporting layers (ETLs), electron injection layers (EILs), or hole blocking layers (HBLs). For simplicity, any layer between an electrode and the EML may be referred to more generally as a charge transport layer (CTL). The CTLs in general operate to transport and inject electrons and holes into the emissive layer, where the electrons and holes recombine to produce light.
Such a light-emitting device in which the material that emits light is organic may be referred to as an organic LED (OLED). Such a light-emitting device in which the material that emits light is semiconductor quantum dots (QDs) may be referred to as a quantum dot LED (QD-LED, QLED or ELQLED).
In device configurations in which all the layers in the light-emitting device are planar, the refractive indices of the layers determine the proportion of the light generated in the EML that can be usefully outcoupled, i.e. emitted from the device into air ultimately to be received by a viewer or external device. Commonly used CTLs have a refractive index in the range 1.7-1.8, which limits the maximum outcoupling efficiency to approximately 20%. Increasing the outcoupling efficiency is desirable because it enables more efficient overall devices, decreasing power consumption and extending device lifetime.
One existing method to improve outcoupling efficiency is to create an optical cavity structure, also known as a microcavity, between the two electrodes or between an electrode and the device substrate. The microcavity may improve the light output coupling, modify the angular distribution of the emitted light, and/or modify the spectral properties of the emitted light such as the full width at half maximum (FWHM) of the emitted spectrum. However, the use of a microcavity structure to enhance light output coupling undesirably results in an increased change in the colour of the light with viewing angle because there is a shift in the emission spectrum to shorter wavelengths for larger viewing angles. A change in colour of the output light from the light-emitting device is an undesirable property when the device is used in display or lighting applications.
FIG. 1 is a drawing depicting a cross-sectional view of a conventional light-emitting device structure 100, such as an OLED or QD-LED. A stack of planar layers is disposed on a substrate 101, with the layers including: two electrodes including a cathode 102 and an anode 103, an emissive layer (EML) 104, one or more charge transport layers (CTL) 105 between the cathode and the EML, and one or more charge transport layers 106 between the anode and the EML. During operation, a bias is applied between the anode and the cathode. The cathode 102 injects electrons into the adjacent CTL 105, and likewise the anode 103 injects holes into the adjacent CTL 106. The electrons and holes propagate through the CTLs to the EML, where they radiatively recombine and light is emitted.
The emitted light may be outcoupled from the device into air, trapped within the layer stack, trapped within the substrate, or trapped within the electrodes as surface plasmons. Light which is trapped within the layer stack or within the substrate may eventually be absorbed. Only light that outcouples into air may be received by an external viewer or device, and therefore only this light contributes to the overall efficiency of the device 100. The device as described with reference to FIG. 1 may be referred to as a “standard” structure in that the anode is closest to the substrate relative to the cathode. However, the positions of the anode and cathode may be interchanged, and comparable principles are equally applicable to either structure. A device in which the cathode is closest to the substrate may be referred to as an “inverted” structure.
As light is generated in the EML and propagates through the layer stack, reflection will occur at interfaces between the different layers due to differences in optical properties, and particularly refractive index, as between the different layers. The EML and CTLs typically have similar refractive indices, and accordingly reflection at these interfaces is very weak. However, in configurations in which reflective or partially reflective electrodes are used, which typically is preferred, the optical properties of the CTLs differ significantly from optical properties of the adjacent electrode layers. Accordingly, a substantial amount of the light will be reflected at the CTL/electrode interfaces.
The two electrodes therefore define a microcavity structure around the EML, and the device structure behaves as a Fabry-Pérot etalon. The outcoupling from such a microcavity structure with transparent layers between the electrodes is a function of the wavelength of light and angle of emission, and may be approximated by:
                              I          =                                                    T                2                            ·                              (                                  1                  +                                      R                    1                                    +                                      2                    ⁢                                                                                            R                          1                                                                    ·                      cos                                        ⁢                                                                                  ⁢                                          δ                      1                                                                      )                                                    1              +                                                R                  1                                ⁢                                  R                  2                                            -                              2                ⁢                                                                                                    R                        1                                            ⁢                                              R                        2                                                                              ·                  cos                                ⁢                                                                  ⁢                δ                                                    ,                            (        1        )            where subscripts refer to the first or second electrode, T is the transmission through the electrode, T1=0, R is the reflectivity of the electrode,
            δ      1        =                  φ        1            +                                    4            ⁢            π            ⁢                                                  ⁢            z                    λ                ⁢        cos        ⁢                                  ⁢        β              ,      δ    =                  φ        1            +              φ        2            +                                    4            ⁢            π            ⁢                                                  ⁢            L                    λ                ⁢        cos        ⁢                                  ⁢        β              ,φ is the phase shift on reflection from the electrode, β is the angle of emission in the EML (measured from the surface normal of the first electrode), z is the optical path length (the product of distance and refractive index) between the position the light is emitted from the EML and the first electrode at β=0°, L is the optical path length between the two electrodes at β=0°, and A is the free space wavelength of the emission. The angle of emission from the device in air relative to the surface normal of the device, θ is related to β by sin(θ)=nEML·sin(β) where nEML is the refractive index of the EML at wavelength λ.
The intensity of emission from the microcavity at each θ and λ may be obtained by multiplying the cavity outcoupling function (equation 1) with the free space emission spectrum from the EML. The free space emission spectrum from the EML may be approximated by the photoluminescence (PL) spectrum from a thick EML, or by the electroluminescence (EL) spectrum from the EML in a structure without a strong microcavity, or by another method. The microcavity may be tuned by adjusting the optical distance between the two reflecting interfaces and by adjusting the position of the EML within the microcavity to obtain a cavity outcoupling function which has a local maximum which is at the same wavelength as the peak free space EML emission, thereby maximising the emission from the device.
The cavity outcoupling function, however, changes as a function of angle both because of the change in optical path length of the light through the layer stack, and because of the change in phase shift on reflection at the interface with the reflective electrode. This leads to the local maximum of the cavity outcoupling function moving to shorter wavelengths as the viewing angle increases away from normal incidence)(θ=0°. Because of the change in the cavity outcoupling function, the spectrum of the emitted light changes as a function of viewing angle and the apparent colour of the light changes. The colour of the light can be defined in the CIE 1976 colour space as is known in the art, representing the colour as a pair of coordinates (u′,v′). The colour shift, Δu′v′, may then be quantified by √[(u′θ=0°−u′θ=α)2+(v′θ=0°−v′θ=α)2] where a is an angle substantially away from 0°. For example, α may be 45°, 60° or 80°.
There have been attempts to mitigate this undesirable colour shift. For example, U.S. Pat. No. 8,894,243 (Cho et al., issued Nov. 25, 2014) and U.S. Pat. No. 9,219,250 (Jeong et al., issued Dec. 22, 2015) disclose the use of a micro-patterned film applied to an OLED to reduce colour shift with viewing angle. U.S. Pat. No. 8,957,443 (Hwang et al., issued Feb. 17, 2015) discloses the use of a device in which one electrode is semi-transparent in one region and transparent in another region, thereby creating two different microcavities which together reduce the colour shift of the output light with viewing angle. U.S. Pat. No. 9,692,017 (Kim et al., issued Jun. 27, 2017) discloses a configuration of the microcavity in which the emission at normal incidence is optimised for a wavelength which is slightly shorter than the free space peak emission wavelength of the EML. This reduces colour shift with viewing angle, but the effectiveness is reduced as wavelength increases. WO 2017/205174 (Freier et al., published Nov. 30, 2017) discloses nanopatterning one or more interfaces between layers of an OLED to reduce colour shift with viewing angle. US 2017/0373277 (Noh et al., published Dec. 28, 2017) discloses the use of a “weak” microcavity which broadens the emission spectrum of the output light. This reduces the colour shift with viewing angle but reduces the colour purity of the output light, and this in turn reduces the achievable colour gamut of a display created using light-emitting devices as described.
These prior attempts to mitigate the colour shift, therefore, remain deficient by compromising other aspects of device performance to address the undesirable colour shift, and/or add substantial complexity to the device configuration and requirements of manufacturing.